In spite of the frequency with which young children have diarrhea, the etiology for at least half of cases remains unexplained even though the human reoviruslike agent has been shown to be a common cause of winter vomiting disease in young children. Toxigenic E. coli have been isolated from large numbers of diarrheic children in many parts of the world. In the few studies which have been done in the United States, only small numbers of toxigenic E. coli have been recovered. In a study in Albuquerque in which 10 normal babies were observed for a year for diarrhea, only one of 14 episodes was associated with toxigenic E. coli. However, sera from 300 children from Albuquerque area revealed an age-related increase in the numbers of children with antibodies to heat-labile toxin. By the age of two years, 28% of children sampled had detectable levels of antibody. The objective of the proposed study is to examine normal babies in the first two years of life for the relationship between fecal carriage of toxigenic bacteria, diarrhea and development of antibodies to heatlabile toxin.